Heat oven to 180C/160C fan/gas 4 and put in a baking tray to heat up. Grease 4 x 150ml ramekins, then add some sugar to each and shake to coat before tipping out the excess.
Whisk the egg whites until stiff peaks form, then add the sugar and continue whisking until stiff again. In a big mixing bowl, stir together the custard and crumbled cake. Stir in a quarter of the egg whites, and very gently fold in the rest using a big metal spoon until the mixture isn’t too streaky.
Divide the mixture between the ramekins, running your finger around the top inside of each to leave a ‘gap’ between the mixture and the ramekin just at the very top. Bake for 12-15 mins until risen and golden. Meanwhile, warm the caramel sauce.
Serve the hot soufflés immediately, with jugs of warm caramel sauce for people to pour into theirs.